Add support of macOS systems

[Alkenso]

I've found that build misses support of macOS build out-of-the-box.
Let's add macOS support, it requires quiet few changes, at least for build of the library itself.

[chxdeng]

Just to confirm, do you use xcode for compilation?
Thanks

[Alkenso]

Thanks for reply,

Usually I prefer to use commandline build via make/cmake.
Also it's great to have ability to specify additional compiler flags.

[chxdeng]

This should have been resolved.